Synchronizing timeslots in a wireless communication protocol

ABSTRACT

A method for synchronizing communications in wireless mesh network operating in a process control environment and including a plurality of network devices includes defining a communication timeslot of a predetermined duration, wherein each of the plurality of network devices transmits or receives data only within the communication timeslot generating a network schedule including at least one superframe having repeating superframe cycles each having a number of communication timeslots sequentially numbered relative to a beginning of each cycle, the number of communication timeslots defining a length of the at least one superframe, maintaining an absolute slot number indicative of a number of communication timeslots scheduled since a start time of the wireless network, synchronizing each of the plurality of network devices with respect to a timing of an individual communication timeslot, and synchronizing each of the plurality of network devices with the network schedule based on the absolute slot number.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of the U.S. Provisional Application No.60/911,795, entitled “Routing, Scheduling, Reliable and SecureOperations in a Wireless Communication Protocol” filed Apr. 13, 2007 ,the disclosure of which is hereby expressly incorporated herein byreference.

FIELD OF TECHNOLOGY

The present invention relates generally to wireless communications and,more particularly, to synchronizing communications in a wirelessnetwork.

BACKGROUND

It is known to use standardized communication protocols in the processcontrol industry to enable devices made by different manufacturers tocommunicate with one another in an easy to use and implement manner. Onesuch well known communication standard used in the process controlindustry is the Highway Addressable Remote Transmitter (HART)Communication Foundation protocol, referred to generally as the HARTprotocol. Generally speaking, the HART protocol supports a combineddigital and analog signal on a dedicated wire or set of wires, in whichon-line process signals (such as control signals, sensor measurements,etc.) are provided as an analog current signal (e.g., ranging from 4 to20 milliamps) and in which other signals, such as device data, requestsfor device data, configuration data, alarm and event data, etc., areprovided as digital signals superimposed or multiplexed onto the samewire or set of wires as the analog signal. However, the HART protocolcurrently requires the use of dedicated, hardwired communication lines,resulting in significant wiring needs within a process plant.

There has been a move, in the past number of years, to incorporatewireless technology into various industries including, in some limitedmanner, the process control industry. However, there are significanthurdles in the process control industry that limit the full scaleincorporation, acceptance and use of wireless technology. In particular,the process control industry requires a completely reliable processcontrol network because loss of signals can result in the loss ofcontrol of a plant, leading to catastrophic consequences, includingexplosions, the release of deadly chemicals or gases, etc. For example,Tapperson et al., U.S. Pat. No. 6,236,334 discloses the use of awireless communications in the process control industry as a secondaryor backup communication path or for use in sending non-critical orredundant communication signals. Moreover, there have been many advancesin the use of wireless communication systems in general that may beapplicable to the process control industry, but which have not yet beenapplied to the process control industry in a manner that allows orprovides a reliable, and in some instances completely wireless,communication network within a process plant. U.S. Patent ApplicationPublication Numbers 2005/0213612, 2006/0029060 and 2006/0029061 forexample disclose various aspects of wireless communication technologyrelated to a general wireless communication system.

One factor significantly inhibiting the development and application ofwireless communications in the process control industry is thedifficulty of retrofitting legacy devices for the use with wirelesscommunication networks. In some cases, devices cannot be retrofitted atall and need to be replaced with newer, wireless-ready models. Moreover,many of the supporting installations are similarly rendered obsolete bya transition to wireless communications. In other words, wirelessnetworks cannot easily extend wired networks. An additional challengeparticularly pertinent to the process control industry is the high costof the existing wired installations and the understandable reluctance ofthe operators to completely replace the wired infrastructure with awireless infrastructure. Meanwhile, wireless networks typically requirestationary antennas or access points to transmit and receive radiosignals and may therefore require an expensive infrastructure whichmakes the transition to wireless communications less desirable. Thus,while some operators may recognize the advantages of a wireless approachto process measurement and control, many may be unwilling to dismantlethe existing installations, decommission the wired devices which may befully operational, and purchase wireless devices.

Another factor contributing to the slower than expected proliferation ofwireless standards in the process control industry is the impact on auser, such as a technician or an operator of a process control system.During operation of a typical process control system, users may remotelyaccess individual devices for the purposes of configuring, monitoring,and controlling various functions of the devices. For example, to enableaccess and exchange of information over the HART protocol, devices areassigned unique addresses according to a predefined addressing scheme.Users and the software applications developed for operators andtechnicians in the process control industry have come to rely on anefficient addressing scheme which cannot be supported by the availablewireless standards. Thus, a transition to a wireless standard in aprocess control industry is widely expected to entail adopting a newaddressing scheme, updating the corresponding software applications andproviding additional training to the personnel.

Additionally, some of the existing wireless standards, such as the IEEE802.11(x) WLAN, for example, do not satisfy all of the demands of theprocess control industry. For example, devices communicate both processand control data which may typically have different propagation delayconstraints. In general, some of the critical data exchanged in theprocess control industry may require efficient, reliable and timelydelivery which cannot always be guaranteed by the existing wirelessprotocols. Moreover, because some of the modules used in the processcontrol industry are used to control very sensitive and potentiallydangerous process activities, wireless standards suitable for thisindustry need to provide redundancy in communication paths not readilyavailable in the known wireless networks. Finally, some process controldevices may be sensitive to high power radio signals and may requireradio transmissions to be limited or held at a well controlled powerlevel. Meanwhile, the available wireless standards typically rely onantennas or access points which transmit relatively strong signals tocover large geographic areas.

Similar to wired communication protocols, wireless communicationprotocols are expected to provide efficient, reliable and secure methodsof exchanging information. Of course, much of the methodology developedto address these concerns on wired networks does not apply to wirelesscommunications because of the shared and open nature of the medium.Further, in addition to the typical objectives behind a wiredcommunication protocol, wireless protocols face other requirements withrespect to the issues of interference and co-existence of severalnetworks that use the same part of the radio frequency spectrum. Tocomplicate matters, some wireless networks operate in the part of thespectrum that is unlicensed, or open to the public. Therefore, protocolsservicing such networks must be capable of detecting and resolvingissues related to frequency (channel) contention, radio resource sharingand negotiation, etc.

In the process control industry, developers of wireless communicationprotocols face additional challenges, such as achieving backwardcompatibility with wired devices, supporting previous wired versions ofa protocol, providing transition services to devices retrofitted withwireless communicators, and providing routing techniques which canensure both reliability and efficiency. Meanwhile, there remains a widenumber of process control applications in which there are few, if any,in-place measurements. Currently these applications rely on observedmeasurements (e.g. water level is rising) or inspection (e.g. periodmaintenance of air conditioning unit, pump, fan, etc.) to discoverabnormal situations. In order to take action, operators frequentlyrequire face-to-face discussions. Many of these applications could begreatly simplified if measurement and control devices were utilized.However, current measurement devices usually require power,communications infrastructure, configuration, and support infrastructurewhich simply is not available.

In yet another aspect, the process control industry requires that thecommunication protocol servicing a particular process control network beable to accommodate field devices with different data transmissionrequirements, priorities, and power capabilities. In particular, someprocess control systems may include measurement devices that frequently(such as several times per second) report measurements to a centralizedcontroller or to another field device. Meanwhile, another device in thesame system may report measurements, alarms, or other data only once perhour. However, both devices may require that the respective measurementreports propagate to a destination host, such as a controller, aworkstation, or a peer field device, with as little overhead in time andbandwidth as possible.

Still further, precise time synchronization is critical to wirelesscommunication systems in general and to Time Division Multiple Access(TDMA)-based protocols in particular. Because TDMA technologiesgenerally involve transmitting and receiving data within controlled timesegments, both the receivers and the transmitters must be aware of aprecise time when each time segments begins and ends, as well as oftransmission or reception opportunities in a TDMA communication scheme.These and similar challenges are particularly prevalent in theenvironments which involve devices transmitting only occasionally.

In another aspect, time synchronization is essential to proper operationof a TDMA communication scheme. Regardless of which hardware time source(e.g., crystals, ceramic resonators etc.) a particular device uses, someskew between the communicating devices (e.g., due to temperature orvoltage variations or ageing) is inevitable. In a process controlindustry, where many devices are frequently exposed to heat or pressure,devices may often lose synchronization.

SUMMARY

A wireless mesh network for use in, for example, process control plantsincludes a plurality of network devices communicating according to anetwork schedule defined as a set of concurrent overlapping superframes.Each superframe includes several communication timeslots of apredetermined duration and each superframe repeats immediately as a newsuperframe cycle after the occurrence of all communication timeslots inthe previous superframe cycle. The timeslots within each superframecycle are sequentially numbered relative to the beginning of the cycle.The wireless mesh network additionally maintains an Absolute Slot Number(ASN) indicative of a number of timeslots scheduled since the time offormation of the wireless network.

In some embodiments, a dedicated service defines superframes andallocates timeslots within each of the superframes according to theneeds of network devices and of external hosts communicating with thenetwork devices. If desired, a network device may participate inmultiple superframes to transmit data specific to the network device andto forward data between other network devices. Optionally, the dedicatedservice may dynamically create and destroy superframes in view ofchanges in network conditions such as data bursts, congestion, blocktransfers, and network devices entering or leaving the network.Moreover, a network device or the dedicated service may efficientlydeactivate a superframe without destroying the superframe by issuing aparticular command. Further, the network schedule may include multiplecommunication channels and, in some embodiments, each communicationchannel may correspond to a unique carrier radio frequency. Each networkdevice may have an individual schedule that includes relative timeslotnumbers and communication channel identifiers and the individualschedule may specify the individually scheduled timeslots that thenetwork device uses to transmit process data, route data originated fromanother network device, receive device-specific data, or receivebroadcast data. In some embodiments, the individual schedule for anetwork device may specify a timeslot associated with several distinctcommunication channels during different superframe cycles, so that thenetwork device transmits or receives data over different communicationchannels within a timeslot having the same relative slot number of aparticular superframe.

In another aspect, a device joining the wireless network may discover acurrent value of the ASN from the potential neighbors of the joiningdevice. Upon extracting the ASN from a corresponding message, thejoining device may determine the current timeslot (i.e., a “relativetimeslot”) in each of the scheduled superframes. As a result, thejoining device may begin to participate in the communication scheme ofthe wireless schedule at one of the predefined timeslots, therebyreducing the probability of collisions and simplifying the resolution ofresource contention.

In yet another aspect, a device may suspend communications with thecommunication network without necessarily destroying one or moresuperframes reserved for publishing the update data of the device. Thedevice may wish to re-join the wireless network at a later time and, inorder to determine when re-joining device may resume publishing thecorresponding process data, the re-joining device may check the ASN todetermine each of the relative slot numbers relevant to the individualschedule of the re-joining device. In some embodiments, a re-joiningdevice may execute an ASN-based resynchronization procedure after atemporary suspension from the wireless network by another device or asoftware entity such as a network manager, for example.

In some embodiments, network devices may additionally transmit a portionof the ASN number, or an “ASN snippet,” to provide diagnosticsinformation. In one such embodiment, each network device includes theASN snippet in every data unit associated with the network layer of thewireless protocol (NPDU). In some embodiments, the ASN snippet includesseveral least significant bits of the precise ASN value. In someembodiments, network devices may use the ASN snippet to calculate theage of a data packet propagating through the wireless network.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that illustrates a system utilizing aWirelessHART network to provide wireless communication between fielddevices and router devices, which are connected to a plant automationnetwork via a gateway device.

FIG. 2 is a schematic representation of the layers of a WirelessHARTprotocol implemented in accordance with one of the embodiments discussedherein.

FIG. 3 is a block diagram that illustrates segments of a communicationtimeslot defined in accordance with one of the embodiments discussedherein.

FIG. 4 is a block diagram that illustrates an exemplary association oftimeslots of a three-slot superframe with several communicating devices.

FIG. 5 schematically illustrates association of a timeslot of anexemplary superframe with several communication channels.

FIG. 6 is a block diagram that schematically illustrates an exemplarysuperframe definition including several concurrent superframes ofdifferent length.

FIG. 7 is another block diagram that schematically illustrates severalconcurrent superframes of different length in relation to an absoluteslot number counter.

FIG. 8 illustrates an example state machine which a network device mayexecute when synchronizing with the wireless network of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary network 10 in which the synchronizationtechniques described herein may be used. In particular, the network 10may include a plant automation network 12 connected to a wirelesscommunication network 14. The plant automation network 12 may includeone or more stationary workstations 16 and one or more portableworkstations 18 connected over a communication backbone 20 which may beimplemented using Ethernet, RS-485, Profibus DP, or using other suitablecommunication hardware and protocol. The workstations and otherequipment forming the plant automation network 12 may provide variouscontrol and supervisory functions to plant personnel, including accessto devices in the wireless network 14. The plant automation network 12and the wireless network 14 may be connected via a gateway device 22.More specifically, the gateway device 22 may be connected to thebackbone 20 in a wired manner and may communicate with the plantautomation network 12 using any suitable (e.g., known) communicationprotocol. The gateway device 22, which may be implemented in any otherdesired manner (e.g., as a standalone device, a card insertable into anexpansion slot of the host workstations 16 or 18, as a part of theinput/output (IO) subsystem of a PLC-based or DCS-based system, etc.),may provide applications that are running on the network 12 with accessto various devices of the wireless network 14. In addition to protocoland command conversion, the gateway device 22 may provide synchronizedclocking used by time slots and superframes (sets of communication timeslots spaced equally in time) of a scheduling scheme associated with awireless protocol (referred to herein as a WirelessHART protocol)implemented in the network 14.

In some configurations, the network 10 may include more than one gatewaydevice 22 to improve the efficiency and reliability of the network 10.In particular, multiple gateway devices 22 may provide additionalbandwidth for the communication between the wireless network 14 and theplant automation network 12, as well as the outside world. On the otherhand, the gateway 22 device may request bandwidth from the appropriatenetwork service according to the gateway communication needs within thewireless network 14. A network manager software module 27, which mayreside in the gateway device 22, may further reassess the necessarybandwidth while the system is operational. For example, the gatewaydevice 22 may receive a request from a host residing outside of thewireless network 14 to retrieve a large amount of data. The gatewaydevice 22 may then request the network manager 27 to allocate additionalbandwidth to accommodate this transaction. For example, the gatewaydevice 22 may issue an appropriate service request. The gateway device22 may then request the network manager 27 to release the bandwidth uponcompletion of the transaction.

In general, the network manager 27 may be responsible for adapting thewireless network 14 to changing conditions and for schedulingcommunication resources. As network devices join and leave the network,the network manager 27 may update its internal model of the wirelessnetwork 14 and use this information to generate communication schedulesand communication routes. Additionally, the network manager 27 mayconsider the overall performance of the wireless network 14 as well asthe diagnostic information to adapt the wireless network 14 to changesin topology and communication requirements. Once the network manager 27has generated the overall communication schedule, all or respectiveparts of the overall communication schedule may be transferred through aseries of commands from the network manager 27 to the network devices.

To further increase bandwidth and improve reliability, the gatewaydevice 22 may be functionally divided into a virtual gateway 24 and oneor more network access points 25A-B, which may be separate physicaldevices in wired communication with the gateway device 22. However,while FIG. 1 illustrates a wired connection 26 between the physicallyseparate gateway device 22 and the access points 25A-B, it will beunderstood that the elements 22-26 may also be provided as an integraldevice. Because the network access points 25A-B may be physicallyseparated from the gateway device 22, the access points 25A-B may bestrategically placed in several different locations with respect to thenetwork 14. In addition to increasing the bandwidth, multiple accesspoints 25A-B can increase the overall reliability of the network 14 bycompensating for a potentially poor signal quality at one access point25A using the other access point 25B. Having multiple access points25A-B also provides redundancy in case of a failure at one or more ofthe access points 25A-B.

In addition to allocating bandwidth and otherwise bridging the networks12 and 14, the gateway device 22 may perform one or more managerialfunctions in the wireless network 14. As illustrated in FIG. 1, anetwork manager software module 27 and a security manager softwaremodule 28 may be stored in and executed in the gateway device 22.Alternatively, the network manager 27 and/or the security manager 28 mayrun on one of the hosts 16 or 18 in the plant automation network 12. Forexample, the network manager 27 may run on the host 16 and the securitymanager 28 may run on the host 18. The network manager 27 may beresponsible for configuration of the network 14, schedulingcommunication between wireless devices, managing routing tablesassociated with the wireless devices, monitoring the overall health ofthe wireless network 14, reporting the health of the wireless network 14to the workstations 16 and 18, as well as other administrative andsupervisory functions. Although a single active network manager 27 maybe sufficient in the wireless network 14, redundant network managers 27may be similarly supported to safeguard the wireless network 14 againstunexpected equipment failures. Meanwhile, the security manager 28 may beresponsible for protecting the wireless network 14 from malicious oraccidental intrusions by unauthorized devices. To this end, the securitymanager 28 may manage authentication codes, verify authorizationinformation supplied by devices attempting to join the wireless network14, update temporary security data such as expiring secret keys, andperform other security functions.

With continued reference to FIG. 1, the wireless network 14 may includeone or more field devices 30-36. In general, process control systems,like those used in chemical, petroleum or other process plants, includesuch field devices as valves, valve positioners, switches, sensors(e.g., temperature, pressure and flow rate sensors), pumps, fans, etc.Field devices perform physical control functions within the process suchas opening or closing valves or take measurements of process parameters.In the wireless communication network 14, field devices 30-36 areproducers and consumers of wireless communication packets.

The devices 30-36 may communicate using a wireless communicationprotocol that provides the functionality of a similar wired network,with similar or improved operational performance. In particular, thisprotocol may enable the system to perform process data monitoring,critical data monitoring (with the more stringent performancerequirements), calibration, device status and diagnostic monitoring,field device troubleshooting, commissioning, and supervisory processcontrol. The applications performing these functions, however, typicallyrequire that the protocol supported by the wireless network 14 providefast updates when necessary, move large amounts of data when required,and support network devices which join the wireless network 14, even ifonly temporarily for commissioning and maintenance work.

In one embodiment, the wireless protocol supporting network devices30-36 of the wireless network 14 is an extension of the known wired HARTprotocol, a widely accepted industry standard, that maintains the simpleworkflow and practices of the wired environment. In this sense, thenetwork devices 30-36 may be considered WirelessHART devices. The sametools used for wired HART devices may be easily adapted to wirelessdevices 30-36 with a simple addition of new device description files. Inthis manner, the wireless protocol may leverage the experience andknowledge gained using the wired HART protocol to minimize training andsimplify maintenance and support. Generally speaking, it may beconvenient to adapt a protocol for wireless use so that mostapplications running on a device do not “notice” the transition from awired network to a wireless network. Clearly, such transparency greatlyreduces the cost of upgrading networks and, more generally, reduces thecost associated with developing and supporting devices that may be usedwith such networks. Some of the additional benefits of a wirelessextension of the well-known HART protocol include access to measurementsthat were difficult or expensive to obtain with wired devices and theability to configure and operate instruments from system software thatcan be installed on laptops, handhelds, workstations, etc. Anotherbenefit is the ability to send diagnostic alerts from wireless devicesback through the communication infrastructure to a centrally locateddiagnostic center. For example, every heat exchanger in a process plantcould be fitted with a WirelessHART device and the end user and suppliercould be alerted when a heat exchanger detects a problem. Yet anotherbenefit is the ability to monitor conditions that present serious healthand safety problems. For example, a WirelessHART device could be placedin flood zones on roads and be used to alert authorities and driversabout water levels. Other benefits include access to a wide range ofdiagnostics alerts and the ability to store trended as well ascalculated values at the WirelessHART devices so that, whencommunications to the device are established, the values can betransferred to a host. In this manner, the WirelessHART protocol canprovide a platform that enables host applications to have wirelessaccess to existing HART-enabled field devices and the WirelessHARTprotocol can support the deployment of battery operated, wireless onlyHART-enabled field devices. The WirelessHART protocol may be used toestablish a wireless communication standard for process applications andmay further extend the application of HART communications and thebenefits that this protocol provides to the process control industry byenhancing the basic HART technology to support wireless processautomation applications.

Referring again to FIG. 1, the field devices 30-36 may be WirelessHARTfield devices, each provided as an integral unit and supporting alllayers of the WirelessHART protocol stack. For example, in the network14, the field device 30 may be a WirelessHART flow meter, the fielddevices 32 may be WirelessHART pressure sensors, the field device 34 maybe a WirelessHART valve positioner, and the field device 36 may aWirelessHART pressure sensor. Importantly, the wireless devices 30-36may support all of the HART features that users have come to expect fromthe wired HART protocol. As one of ordinary skill in the art willappreciate, one of the core strengths of the HART protocol is itsrigorous interoperability requirements. In some embodiments, allWirelessHART equipment includes core mandatory capabilities in order toallow equivalent device types (made by different manufacturers, forexample) to be interchanged without compromising system operation.Furthermore, the WirelessHART protocol is backward compatible to HARTcore technology such as the device description language (DDL). In thepreferred embodiment, all of the WirelessHART devices should support theDDL, which ensures that end users immediately have the tools to beginutilizing the WirelessHART protocol.

If desired, the network 14 may include non-wireless devices. Forexample, a field device 38 of FIG. 1 may be a legacy 4-20 mA device anda field device 40 may be a traditional wired HART device. To communicatewithin the network 14, the field devices 38 and 40 may be connected tothe WirelessHART network 14 via a WirelessHART adaptor (WHA) 50.Additionally, the WHA 50 may support other communication protocols suchas Foundation® Fieldbus, PROFIBUS, DevicesNet, etc. In theseembodiments, the WHA 50 supports protocol translation on a lower layerof the protocol stack. Additionally, it is contemplated that a singleWHA 50 may also function as a multiplexer and may support multiple HARTor non-HART devices.

Plant personnel may additionally use handheld devices for installation,control, monitoring, and maintenance of network devices. Generallyspeaking, handheld devices are portable equipment that can connectdirectly to the wireless network 14 or through the gateway devices 22 asa host on the plant automation network 12. As illustrated in FIG. 1, aWirelessHART-connected handheld device 55 may communicate directly withthe wireless network 14. When operating with a formed wireless network14, the handheld device 55 may join the network 14 as just anotherWirelessHART field device. When operating with a target network devicethat is not connected to a WirelessHART network, the handheld device 55may operate as a combination of the gateway device 22 and the networkmanager 27 by forming its own wireless network with the target networkdevice.

A plant automation network-connected handheld device (not shown) may beused to connect to the plant automation network 12 through knownnetworking technology, such as Wi-Fi. This device communicates with thenetwork devices 30-40 through the gateway device 22 in the same fashionas external plant automation servers (not shown) or the workstations 16and 18 communicate with the devices 30-40.

Additionally, the wireless network 14 may include a router device 60which is a network device that forwards packets from one network deviceto another network device. A network device that is acting as a routerdevice uses internal routing tables to conduct routing, i.e., to decideto which network device a particular packet should be sent. Standalonerouters such as the router 60 may not be required in those embodimentswhere all of the devices on the wireless network 14 support routing.However, it may be beneficial (e.g. to extend the network, or to savethe power of a field device in the network) to add one or more dedicatedrouters 60 to the network 14.

All of the devices directly connected to the wireless network 14 may bereferred to as network devices. In particular, the wireless fielddevices 30-36, the adapters 50, the routers 60, the gateway devices 22,the access points 25A-B, and the wireless handheld device 55 are, forthe purposes of routing and scheduling, network devices, each of whichforms a node of the wireless network 14. In order to provide a veryrobust and an easily expandable wireless network, all of the devices ina network may support routing and each network device may be globallyidentified by a substantially unique address, such as a HART address,for example. The network manager 27 may contain a complete list ofnetwork devices and may assign each device a short, network unique16-bit nickname. Additionally, each network device may store informationrelated to update rates, connection sessions, and device resources. Inshort, each network device maintains up-to-date information related torouting and scheduling within the wireless network 14. The networkmanager 27 may communicate this information to network devices whenevernew devices join the network or whenever the network manager 27 detectsor originates a change in topology or scheduling of the wireless network14.

Further, each network device may store and maintain a list of neighbordevices that the network device has identified during listeningoperations. Generally speaking, a neighbor of a network device isanother network device of any type potentially capable of establishing aconnection with the network device in accordance with the standardsimposed by a corresponding network. In case of the WirelessHART network14, the connection is a direct wireless connection. However, it will beappreciated that a neighboring device may also be a network deviceconnected to the particular device in a wired manner. As will bediscussed later, network devices promote their discovery by othernetwork devices through advertisement, or special messages sent outduring designated periods of time. Network devices operatively connectedto the wireless network 14 have one or more neighbors which they maychoose according to the strength of the advertising signal or to someother principle.

In the example illustrated in FIG. 1, each of a pair of network devicesthat are connected by a direct wireless connection 65 recognizes theother as a neighbor. Thus, network devices of the wireless network 14may form a large number of inter-device connections 65. The possibilityand desirability of establishing a direct wireless connection 65 betweentwo network devices is determined by several factors, such as thephysical distance between the nodes, obstacles between the nodes(devices), signal strength at each of the two nodes, etc. Further, twoor more direct wireless connections 65 may be used to form communicationpaths between nodes that cannot form a direct wireless connection 65.For example, the direct wireless connection 65 between the WirelessHARThand-held device 55 and WirelessHART device 36 along with the directwireless connection 65 between the WirelessHART device 36 the router 60form a communication path between the devices 55 and 60.

Each wireless connection 65 is characterized by a large set ofparameters related to the frequency of transmission, the method ofaccess to a radio resource, etc. One of ordinary skill in the art willrecognize that, in general, wireless communication protocols may operateon designated frequencies, such as the ones assigned by the FederalCommunications Commission (FCC) in the United States, or in theunlicensed part of the radio spectrum (e.g., 2.4 GHz). While the systemand method discussed herein may be applied to a wireless networkoperating on any designated frequency or range of frequencies, theexample embodiment discussed below relates to the wireless network 14operating in the unlicensed, or shared part of the radio spectrum. Inaccordance with this embodiment, the wireless network 14 may be easilyactivated and adjusted to operate in a particular unlicensed frequencyrange as needed.

One of the core requirements for a wireless network protocol using anunlicensed frequency band is the minimally disruptive coexistence withother equipment utilizing the same band. Coexistence generally definesthe ability of one system to perform a task in a shared environment inwhich other systems can similarly perform their tasks while conformingto the same set of rules or to a different (and possibly unknown) set ofrules. One requirement of coexistence in a wireless environment is theability of the protocol to maintain communication while interference ispresent in the environment. Another requirement is that the protocolshould cause as little interference and disruption as possible withrespect to other communication systems.

In other words, the problem of coexistence of a wireless system with thesurrounding wireless environment has two general aspects. The firstaspect of coexistence is the manner in which the system affects othersystems. For example, an operator or developer of the particular systemmay ask what impact the transmitted signal of one transmitter has onother radio system operating in proximity to the particular system. Morespecifically, the operator may ask whether the transmitter disruptscommunication of some other wireless device every time the transmitterturns on or whether the transmitter spends excessive time on the aireffectively “hogging” the bandwidth. Ideally, each transmitter should bea “silent neighbor” that no other transmitter notices. While this idealcharacteristic is rarely, if ever, attainable, a wireless system thatcreates a coexistence environment in which other wireless communicationsystems may operate reasonably well may be called a “good neighbor.” Thesecond aspect of coexistence of a wireless system is the ability of thesystem to operate reasonably well in the presence of other systems orwireless signal sources. In particular, the robustness of a wirelesssystem may depend on how well the wireless system prevents interferenceat the receivers, on whether the receivers easily overload due toproximate sources of RF energy, on how well the receivers tolerate anoccasional bit loss, and similar factors. In some industries, includingthe process control industry, there are a number of important potentialapplications in which the loss of data is frequently not allowable. Awireless system capable of providing reliable communications in a noisyor dynamic radio environment may be called a “tolerant neighbor.”

Effective coexistence (i.e., being a good neighbor and a tolerantneighbor) relies in part on effectively employing three aspects offreedom: time, frequency and distance. Communication can be successfulwhen it occurs 1) at a time when the interference source (or othercommunication system) is quiet; 2) at a different frequency than theinterference signal; or 3) at a location sufficiently removed from theinterference source. While a single one of these factors could be usedto provide a communication scheme in the shared part of the radiospectrum, a combination of two or all three of these factors can providea high degree of reliability, security and speed.

Still referring to FIG. 1, the network manager 27 or another applicationor service running on the network 14 or 12 may define a master networkschedule 67 for the wireless communication network 14 in view of thefactors discussed above. The master network schedule 67 may specify theallocation of resources such as time segments and radio frequencies tothe network devices 25A-B and 30-55. In particular, the master networkschedule 67 may specify when each of the network devices 25A-B and 30-55transmits process data, routes data on behalf of other network devices,listens to management data propagated from the network manager 27, andtransmits advertisement data for the benefit of devices wishing to jointhe wireless network 14. To allocate the radio resources in an efficientmanner, the network manager 27 may define and update the master networkschedule 67 in view of the topology of the wireless network 14. Morespecifically, the network manager 27 may allocate the availableresources to each of the nodes of the wireless network 14 (i.e.,wireless devices 30-36, 50, and 60) according to the direct wirelessconnections 65 identified at each node. In this sense, the networkmanager 27 may define and maintain the network schedule 67 in view ofboth the transmission requirements and of the routing possibilities ateach node.

The master network schedule 67 may partition the available radio sourcesinto individual communication channels, and further measure transmissionand reception opportunities on each channel in such units as TimeDivision Multiple Access (TDMA) communication timeslots, for example. Inparticular, the wireless network 14 may operate within a certainfrequency band which, in most cases, may be safely associated withseveral distinct carrier frequencies, so that communications at onefrequency may occur at the same time as communications at anotherfrequency within the band. One of ordinary skill in the art willappreciate that carrier frequencies in a typical application (e.g.,public radio) are sufficiently spaced apart to prevent interferencebetween the adjacent carrier frequencies. For example, in the 2.4 GHzband, IEEE assigns frequency 2.455 to channel number 21 and frequency2.460 to channel number 22, thus allowing the spacing of 5 KHz betweentwo adjacent segments of the 2.4 GHz band. The master network schedule67 may thus associate each communication channel with a distinct carrierfrequency, which may be the center frequency in a particular segment ofthe band.

Meanwhile, as typically used in the industries utilizing TDMAtechnology, the term “timeslot” refers to a segment of a specificduration into which a larger period of time is divided to provide acontrolled method of sharing. For example, a second may be divided into10 equal 100 millisecond timeslots. Although the master network schedule67 preferably allocates resources as timeslots of a single fixedduration, it is also possible to vary the duration of the timeslots,provided that each relevant node of the wireless network 14 is properlynotified of the change. To continue with the example definition of ten100-millisecond timeslots, two devices may exchange data every second,with one device transmitting during the first 100 ms period of eachsecond (i.e., the first timeslot), the other device transmitting duringthe fourth 100 ms period of each second (i.e., the fourth timeslot), andwith the remaining timeslots being unoccupied. Thus, a node on thewireless network 14 may identify the scheduled transmission or receptionopportunity by the frequency of transmission and the timeslot duringwhich the corresponding device may transmit or receive data.

To properly synchronize the network devices 25A-B and 30-50 with themaster network schedule 67, the network manager 27 may maintain acounter 68 to keep track of a number of timeslots scheduled since theformation of the wireless network 14, i.e., since a first network deviceinitiated the process of forming the wireless network 14. As indicatedabove, the first network device may be the gateway device 22, forexample. The number of timeslots elapsed since the beginning of thewireless network 14 is referred to herein as the Absolute Slot Number(“ASN”), in contrast to a relative slot number of a timeslot in aparticular superframe. The network manager 27 may initialize the ASNcounter 68 to zero at the time of formation of the wireless network 14and increment consequently increment the ASN counter 68 by one with eachoccurrence of a new timeslot. As discussed in greater detail below, eachof the network devices 25A-B and 30-50 may similarly maintain a localcopy of the ASN counter 68 and periodically synchronize the local copywith the master ASN counter 68 maintained by the network manager 27 orthe gateway 22 (e.g., the network manager 27 and the gateway 22 mayshare this information through a private interface).

As part of defining an efficient and reliable network schedule 67, thenetwork manager 27 may logically organize timeslots into cyclicallyrepeating sets, or superframes. As used herein, a superframe may be moreprecisely understood as a series of equal superframe cycles, eachsuperframe cycle corresponding to a logical grouping of several adjacenttime slots forming a contiguous segment of time. The number of timeslots in a given superframe defines the length of the superframe anddetermines how often each time slot repeats. In other words, the lengthof a superframe, multiplied by the duration of a single timeslot,specifies the duration of a superframe cycle. Additionally, thetimeslots within each frame cycle may be sequentially numbered forconvenience. To take one specific example, the network manager 27 mayfix the duration of a timeslot at 10 milliseconds and may define asuperframe of length 100 to generate a 1-second frame cycle (i.e., 10milliseconds multiplied by 100). In a zero-based numbering scheme, thisexample superframe may include timeslots numbered 0, 1, . . . 99.

As discussed in greater detail below, the network manager 27 reduceslatency and otherwise optimizes data transmissions by including multipleconcurrent superframes of different sizes in the network schedule 67.Moreover, some or all of the superframes of the network schedule 67 mayspan multiple channels, or carrier frequencies. Thus, the master networkschedule 67 may specify the association between each timeslot of eachsuperframe and one of the available channels.

Thus, the master network schedule 67 may correspond to an aggregation ofindividual device schedules. For example, a network device, such as thevalve positioner 34, may have an individual device schedule 69A. Thedevice schedule 69A may include only the information relevant to thecorresponding network device 34. Similarly, the router device 60 mayhave an individual device schedule 69B. Accordingly, the network device34 may transmit and receive data according to the device schedule 69Awithout knowing the schedules of other network devices such as theschedule 69B of the device 60. To this end, the network manager 27 maymanage both the overall network schedule 67 and each of the individualdevice schedules 69 (e.g., 69A and 69B) and communicate the individualdevice schedules 69 to the corresponding devices when necessary. Inother embodiments, the individual network devices 25A-B and 35-50 may atleast partially define or negotiate the device schedules 69 and reportthese schedules to the network manager 27. According to this embodiment,the network manager 27 may assemble the network schedule 67 from thereceived device schedules 69 while checking for resource contention andresolving potential conflicts.

The communication protocol supporting the wireless network 14 generallydescribed above is referred to herein as the WirelessHART protocol 70,and the operation of this protocol is discussed in more detail withrespect to FIG. 2. As will be understood, each of the direct wirelessconnections 65 may transfer data according to the physical and logicalrequirements of the WirelessHART protocol 70. Meanwhile, theWirelessHART protocol 70 may efficiently support communications withintimeslots and on the carrier frequencies associated with the superframesdefined by the device-specific schedules 69.

FIG. 2 schematically illustrates the layers of one example embodiment ofthe WirelessHART protocol 70, approximately aligned with the layers ofthe well-known ISO/OSI 7-layer model for communications protocols. Byway of comparison, FIG. 2 additionally illustrates the layers of theexisting “wired” HART protocol 72. It will be appreciated that theWirelessHART protocol 70 need not necessarily have a wired counterpart.However, as will be discussed in detail below, the WirelessHART protocol70 can significantly improve the convenience of its implementation bysharing one or more upper layers of the protocol stack with an existingprotocol. As indicated above, the WirelessHART protocol 70 may providethe same or greater degree of reliability and security as the wiredprotocol 72 servicing a similar network. At the same time, byeliminating the need to install wires, the WirelessHART protocol 70 mayoffer several important advantages, such as the reduction of costassociated with installing network devices, for example. It will be alsoappreciated that although FIG. 2 presents the WirelessHART protocol 70as a wireless counterpart of the HART protocol 72, this particularcorrespondence is provided herein by way of example only. In otherpossible embodiments, one or more layers of the WirelessHART protocol 70may correspond to other protocols or, as mentioned above, theWirelessHART protocol 70 may not share even the uppermost applicationlayer with any of the existing protocols.

As illustrated in FIG. 2, the wireless expansion of HART technology mayadd at least one new physical layer (e.g., the IEEE 802.15.4 radiostandard) and two data-link layers (e.g., wired and wireless mesh) tothe known HART implementation. In general, the WirelessHART protocol 70may be a secure, wireless mesh networking technology operating in the2.4 GHz ISM radio band (block 74). In one embodiment, the WirelessHARTprotocol 70 may utilize IEEE 802.15.4b compatible direct sequence spreadspectrum (DSSS) radios with channel hopping on a transaction bytransaction basis. This WirelessHART communication may be arbitratedusing TDMA to schedule link activity (block 76). As such, allcommunications are preferably performed within a designated time slot.One or more source and one or more destination devices may be scheduledto communicate in a given slot, and each slot may be dedicated tocommunication from a single source device, or the source devices may bescheduled to communicate using a CSMA/CA-like shared communicationaccess mode. Source devices may send messages to one or more specifictarget devices or may broadcast messages to all of the destinationdevices assigned to a slot.

Because the WirelessHART protocol described herein allows deployment ofmesh topologies, a significant network layer 78 may be specified aswell. In particular, the network layer 78 may enable establishing directwireless connections 65 between individual devices and routing databetween a particular node of the wireless network 14 (e.g., the device34) and the gateway 22 via one or more intermediate hops. In someembodiments, pairs of network devices 25A-B and 30-50 may establishcommunication paths including one or several hops while in otherembodiments, all data may travel either upstream to the gateway device22 or downstream from the gateway device 22 to a particular node.

To enhance reliability, the WirelessHART protocol 70 may combine TDMAwith a method of associating multiple radio frequencies with a singlecommunication resource, e.g., channel hopping. Channel hopping providesfrequency diversity which minimizes interference and reduces multi-pathfading effects. In particular, the data link 76 may create anassociation between a single superframe and multiple carrier frequencieswhich the data link layer 76 cycles through in a controlled andpredefined manner. For example, the available frequency band of aparticular instance of the WirelessHART network 14 may have carrierfrequencies F₁, F₂, . . . F_(n). A relative frame R of a superframe Smay be scheduled to occur at a frequency F₁ in the cycle C_(n), at afrequency F₅ in the following cycle C_(n+1), at a frequency F₂ in thecycle C_(n+2), and so on. The network manager 27 may configure therelevant network devices with this information so that the networkdevices communicating in the superframe S may adjust the frequency oftransmission or reception according to the current cycle of thesuperframe S.

The data link layer 76 of the WirelessHART protocol 70 may offer anadditional feature of channel blacklisting, which restricts the use ofcertain channels in the radio band by the network devices. The networkmanager 27 may blacklist a radio channel in response to detectingexcessive interference or other problems on the channel. Further,operators or network administrators may blacklist channels in order toprotect a wireless service that uses a fixed portion of the radio bandthat would otherwise be shared with the WirelessHART network 14. In someembodiments, the WirelessHART protocol 70 controls blacklisting on asuperframe basis so that each superframe has a separate blacklist ofprohibited channels.

In one embodiment, the network manager 27 is responsible for allocating,assigning, and adjusting time slot resources associated with the datalink layer 76. If a single instance of the network manager 27 supportsmultiple WirelessHART networks 14, the network manager 27 may create anoverall schedule for each instance of the WirelessHART network 14. Theschedule may be organized into superframes containing time slotsnumbered relative to the start of the superframe.

The WirelessHART protocol 70 may further define links or link objects inorder to logically unite scheduling and routing. In particular, a linkmay be associated with a specific network device, a specific superframe,a relative slot number, one or more link options (transmit, receive,shared), and a link type (normal, advertising, discovery). Asillustrated in FIG. 2, the data link layer 76 may be frequency-agile.More specifically, a channel offset may be used to calculate thespecific radio frequency used to perform communications. The networkmanager 27 may define a set of links in view of the communicationrequirements at each network device. Each network device may then beconfigured with the defined set of links. The defined set of links maydetermine when the network device needs to wake up, and whether thenetwork device should transmit, receive, or both transmit/receive uponwaking up.

With continued reference to FIG. 2, the transport layer 80 of theWirelessHART protocol 70 allows efficient, best-effort communication andreliable, end-to-end acknowledged communications. As one skilled in theart will recognize, best-effort communications allow devices to senddata packets without an end-to-end acknowledgement and no guarantee ofdata ordering at the destination device. User Datagram Protocol (UDP) isone well-known example of this communication strategy. In the processcontrol industry, this method may be useful for publishing process data.In particular, because devices propagate process data periodically,end-to-end acknowledgements and retries have limited utility, especiallyconsidering that new data is generated on a regular basis. In contrast,reliable communications allow devices to send acknowledgement packets.In addition to guaranteeing data delivery, the transport layer 80 mayorder packets sent between network devices. This approach may bepreferable for request/response traffic or when transmitting eventnotifications. When the reliable mode of the transport layer 80 is used,the communication may become synchronous.

Reliable transactions may be modeled as a master issuing a requestpacket and one or more slaves replying with a response packet. Forexample, the master may generate a certain request and can broadcast therequest to the entire network. In some embodiments, the network manager27 may use reliable broadcast to tell each network device in theWirelessHART network 14 to activate a new superframe. Alternatively, afield device such as the sensor 30 may generate a packet and propagatethe request to another field device such as to the portable HARTcommunicator 55. As another example, an alarm or event generated by the34 field device may be transmitted as a request directed to the gatewaydevice 22. In response to successfully receiving this request, thegateway device 22 may generate a response packet and send the responsepacket to the device 34, acknowledging receipt of the alarm or eventnotification.

Referring again to FIG. 2, the session layer 82 may providesession-based communications between network devices. End-to-endcommunications may be managed on the network layer by sessions. Anetwork device may have more than one session defined for a given peernetwork device. If desired, almost all network devices may have at leasttwo sessions established with the network manager 27: one for pairwisecommunication and one for network broadcast communication from thenetwork manager 27. Further, all network devices may have a gatewaysession key. The sessions may be distinguished by the network deviceaddresses assigned to them. Each network device may keep track ofsecurity information (encryption keys, nonce counters) and transportinformation (reliable transport sequence numbers, retry counters, etc.)for each session in which the device participates.

Finally, both the WirelessHART protocol 70 and the wired HART protocol72 may support a common HART application layer 84. The application layerof the WirelessHART protocol 70 may additionally include a sub-layer 86supporting auto-segmented transfer of large data sets. By sharing theapplication layer 84, the protocols 70 and 72 allow for a commonencapsulation of HART commands and data and eliminate the need forprotocol translation in the uppermost layer of the protocol stack.

FIGS. 3-6 provide a more detailed illustration of channel and timeslotresource allocation supported by the data link layer 76 and the networklayer 78 of the WirelessHART protocol 70. As discussed above inreference to FIG. 1, the network manager 27 may manage the definition ofone or more superframes and may associate individual timeslots withineach of the defined superframes with one of the available channels(e.g., carrier frequencies). By way of one specific example, FIG. 3illustrates a possible communication scheme within an individualtimeslot, while FIG. 4 illustrates an example data exchange betweenseveral devices using the timeslots of a certain superframe. Next, FIG.5 illustrates a possible association between an example timeslot andseveral available channels, and FIG. 6 is a schematic representation ofseveral concurrent superframes which include the timeslots illustratedin FIGS. 3-5.

Referring specifically to FIG. 3, two or mode network devices mayexchange data in a timeslot 100, which may be a dedicated timeslotshared by one transmitting device and one receiving device or a sharedtimeslot having more than one transmitter and/or one or more receivers.In either case, the timeslot 100 may have a transmit schedule 102 and areceive schedule 104. In other words, one or more transmitting devicesmay communicate within the timeslot 100 according to the transmittimeslot schedule 102 while one or more receiving devices maycommunicate within the timeslot 100 according to the receive timeslotschedule 104. Of course, the timeslot schedules 102 and 104 aresubstantially precisely synchronized and begin at the same relative time106. Over the course of the timeslot 100, a transmitting network devicesends a predetermined amount of data over a communication channel suchas a carrier radio frequency. In some cases, the transmitting networkdevice may also expect to receive a positive or negative acknowledgementwithin the same timeslot 100.

Thus, as illustrated in FIG. 3, the transmit timeslot schedule 102 mayinclude a transmit segment 110 for transmitting outbound data, precededby a pre-transmission segment 112, and may include a receive segment 122for receiving an acknowledgement for the data transmitted during thesegment 110. The transmit segment 110 may be separated from the receivesegment 122 by a transition segment 116, during which the correspondingnetwork device may adjust the hardware settings, for example. Meanwhile,the receive schedule 104 may include segments for performing functionscomplementary to those carried out in the segments 112-122, as discussedbelow.

In particular, the transmitting device may send out the entire packet orstream segment associated with a capacity of the timeslot 100 during thesegment 110. As mentioned above, the network schedule 69 may includeshared timeslots which do not exclusively belong to an individual deviceschedule 67 of one of the network devices 25A-B and 30-55. For example,a shared timeslot may have a dedicated receiver such as the gatewaydevice 22 but no single dedicated transmitter. When necessary, one ofthe network devices 25A-60 may transmit unscheduled information, such asa request for additional bandwidth, over the shared timeslot. In thesecases, the potentially transmitting device may check whether the sharedtimeslot is available by performing Clear Channel Assessment (CCA) in apre-transmission segment 112. In particular, the transmitting networkdevice may listen to signals propagated over the communication channelassociated with the timeslot 100 for the duration of thepre-transmission segment 112 to confirm that no other network device isattempting to use the timeslot 100.

On the receiving end of the timeslot 100, the receiving device mayreceive the entire packet associated with the timeslot 100 within apacket receive segment 114. As illustrated in FIG. 3, the packet receivesegment 114 may begin at an earlier point in time than the transmitsegment 110. Next, the transmit timeslot schedule 102 requires that thetransmitting device transition the radio mode in a transition segment116. Similarly, the receive timeslot schedule 104 includes a transitionsegment 118. However, the segment 116 may be shorter than the segment118 because the transmitting device may start listening foracknowledgement data early to avoid missing a beginning of anacknowledgement.

Still further, the transmit schedule 102 may include an acknowledgementreceive segment 122 during which the transmitting device receives anacknowledgement transmitted during an acknowledgement transmit segment124 associated with the receive schedule 104. The transmitting devicemay delete the packet transmitted during the transmit segment 110 froman associated transmit queue upon receiving a positive acknowledgement.On the other hand, the transmitting device may attempt to re-transmitthe packet in the next scheduled dedicated timeslot or in the nextavailable shared timeslot if no acknowledgement arrives or if theacknowledgement is negative.

Several timeslots 100 discussed above may be organized into a superframe140, as schematically illustrated in FIG. 4. In particular, thesuperframe 140 may include a (typically) infinite series of superframecycles 150-154, each cycle including a set if timeslots, illustrated inFIG. 4 as a timeslot 142 with a relative timeslot number 0 (TS0), atimeslot 144 with a relative timeslot number 1 (TS1), and a timeslot 146with a relative timeslot number 2 (TS2). Accordingly, the size of thesuperframe 140 of FIG. 4 is three timeslots. In other words, each of thetimeslots 142-146 of the superframe 140 repeats in time at an intervalof two intermediate timeslots. Thus, for a 10 millisecond timeslot, theinterval between the end of a timeslot with a particular relative slotnumber and the beginning of a next timeslot with the same relative slotnumber is 20 milliseconds. Conceptually, the timeslots 142-146 may befurther grouped into superframe cycles 150-154. As illustrated in FIG.4, each superframe cycle corresponds to a new instance of a sequence oftimeslots 142-146.

The master network schedule 67 may associate transmission and receptionopportunities of some of the network devices participating in thewireless network 14 with particular timeslots of the superframe 140.Referring again to FIG. 4, a network fragment 160 schematicallyillustrates a partial communication scheme implemented between thenetwork devices 34, 60, and 36 of FIG. 1. To simplify the illustrationof the superframe 140, the network devices 34, 60, and 36 areadditionally designed in FIG. 4 as nodes A, B, and C, respectively.Thus, according to FIG. 4, the node A transmits data to the node Bwhich, in turn, transmits data to the node C. As discussed above, eachof the nodes A-C includes a device schedule 69A-C, which specifies thetimeslots and channels (e.g., radio carrier frequencies) fortransmitting and receiving data at the corresponding device. The masternetwork schedule 67 may include part of all of the data informationstored in the individual device schedules 69A-C. More specifically, thenetwork manager 27 may maintain the master network schedule 67 as anaggregate of the schedules associated with each of the network devices25A-B and 30-50, including the device schedules 69A-C.

In this example, the duration of the timeslot 100 (FIG. 3) may be 10milliseconds and the network device A may report data to the device Cevery 30 milliseconds. Accordingly, the network manager 27 may set thelength of the superframe 140 at three timeslots specifically in view ofthe update rate of the network device A. Further, the network manager 27may assign the timeslot 142 with a relative number 0 (TS0) to thenetwork devices A and B, with the device A as the transmitter and thedevice B as the receiver. The network manager 27 may further allocatethe next available timeslot 144, having the relative slot number 1(TS1), to be associated with the transmission from the device B to thedevice C. Meanwhile, the timeslot 146 remains unassigned. In thismanner, the superframe 140 provides a scheme according to which thenetwork manager 27 may allocate resources in the network fragment 160for the transmission of data from the device A to the device C in viewof the available wireless connections between the devices A, B, and C.

In the example illustrated in FIG. 4, the network device at node A maystore information related to the timeslot 142 as part of its deviceschedule 69A. Similarly, the network device at node B may storeinformation related to the timeslots 142 (receive) and 144 (transmit) aspart of its device schedule 69B. Finally, the network device C may storeinformation related to the timeslot 144 in the device schedule 69C. Inat least some of the embodiments, the network manager 27 storesinformation about the entire superframe 140, including an indicationthat the timeslot 146 is available.

Importantly, the superframe 140 need not be restricted to a single radiofrequency or other single communication channel. In other words, theindividual timeslots 142-146 defining the superframe 140 may beassociated with different radio frequencies on a permanent or floatingbasis. Moreover, the frequencies used by the various devices need notalways be adjacent in the electromagnetic spectrum. In one embodiment,for example, the timeslot 142 of each of the superframe cycles 150-154may be associated with a carrier frequency F₁ and the timeslot 144 ofeach of the superframe cycles 150-154 may be associated with a carrierfrequency F₂, with the frequencies F₁ and F₂ being adjacent ornon-adjacent in the electromagnetic spectrum.

In another embodiment, at least some of the timeslots 142-146 may moveabout the allocated frequency band in a predefined manner. FIG. 5illustrates an example association of the timeslot 144 of FIG. 4 withchannels 172-179 (corresponding to frequency sub-bands F₁-F₅) in theavailable frequency band 170. In particular, each of the channels172-179 may correspond to one of the center frequencies F₁, F₂, . . . F₅which preferably differ from their respective neighbors by the sameoffset. The channels 172-179 preferably form a continuous section of thespectrum covering the entire available frequency band 170, although thechannels 172-179 need be contiguous or form a continuous band in allembodiments. The superframe 140 may use at least a portion of thefrequency band 170, so that one or more of the timeslots 142-146 arescheduled on different carrier frequencies in at least two consecutivecycles.

As illustrated in FIG. 5, the timeslot 144 may use the channel 176(frequency F₃) during the frame cycle 150, may use the channel 174(frequency F₄) during the frame cycle 152, and may use the channel 178(frequency F₂) during the frame cycle 154. The timeslot 144 may then“return” to the channel 176 in the next superframe cycle 150A, which maysimilar to the cycle 150. Each of the specific associations of thetimeslot 144 with one of the channels 172-179 is illustrated as atimeslot/channel tuple 144A-C. For example, the tuple 144A specifies thetimeslot 2 scheduled, in the cycle 150, on the channel 176 associatedwith the center frequency F₃. Similarly, the tuple 144B specifies thetimeslot 2 scheduled, in the cycle 152, on the channel 174 associatedwith the center frequency F₄. Meanwhile, the channel 172 associated withthe center frequency F₅ may not be assigned to the timeslot 2 during anyof the cycles 150-152. However, a different timeslot of the superframe140 such as the timeslot 146, for example, may be associated with thechannel 172 during one or more of the cycles 150-152.

In this example, the frequency assignment associated with the superframecycle 150 may repeat immediately following the cycle 154 (illustrated asa cycle 150A in the FIG. 5), and the timeslot 144 may again correspondto the tuple 144A after two cycles of the superframe 140. Thus, thetimeslot 144 may regularly cycle through the channels 176, 174, and 178.It will be appreciated that the timeslot 144 may similarly cycle througha greater or smaller number of channels irrespective of the length ofthe superframe 140, provided, of course, that enough channels areavailable in the frequency band 170. The association of a singletimeslot with multiple channels during different superframe cycles,discussed above with respect to FIG. 5 and referred to herein as“channel hopping,” significantly increases the reliability of thewireless network 14. In particular, channel hopping reduces theprobability that a pair of devices, scheduled to communicate in aparticular timeslot of a certain superframe, fail to transmit andreceive data when a certain channel is jammed or otherwise unavailable.Thus, for example, the failure of the channel 174 prevents the devicesusing the timeslot 144 from communicating in the frame cycle 152 but notduring the frame cycles 150 or 154.

Referring again to FIG. 4, the device schedules 69B and 69C may includethe information regarding each of the tuples 144A-C discussed above inreference to FIG. 5. In particular, each of the device schedules 69B and69C may store an assignment of the timeslot 144 to one of the channels172-179 within each of the cycles 150-152. The master network schedule67 (FIG. 1) may similarly include this information. Meanwhile, thedevice schedule 69A need not necessarily include the information relatedto the timeslot 144 because the corresponding node A (the device 34)does not communicate during the timeslot 144 of the superframe 140. Inoperation, the devices 60 and 36 corresponding to the nodes B and C mayprepare for data transmission and reception, respectively, at thebeginning of each timeslot 144. To determine whether the timeslot 144currently corresponds to the tuple 144A, 144B, or 144C, the devices 60and 36 may apply a locally stored copy of the ASN counter 68 todetermine whether the timeslot 144 is currently in the frame cycle 150,152, or 154.

In the process of defining the network schedule 69, the network manager27 may define multiple concurrent superframes in view of the updaterates of the network devices 25A-B and 35-50. As illustrated in FIG. 6,the network schedule 69 may include the superframe 140 of length threeas well superframes 190 and 192. The superframe 190 may be a five-slotsuperframe and the superframe 192 may be a four-slot superframe,although the different superframes may have a different number of slotsand various different superframes may have the same number of slots. Asillustrated in FIG. 6, the superframes need not necessarily align withrespect to the relative slot numbers. In particular, at a particulartime 194, the superframe 190 may schedule the timeslot with the relativenumber two (TS2) while the superframes 140 and 192 may schedule thetimeslots with the relative number one (TS1). Preferably, thesuperframes 140, 190, and 192 are time-synchronized so that eachtransition to a new timeslot within each of these superframes occurs atthe same time.

Each of the superframes 140, 190 and 192 may be primarily associatedwith, or “belong to” an individual one of or a subset of the networkdevices 25A-B and 30-50. For example, the superframe 140 illustrated inFIG. 4 may belong to the node A (i.e., the network device 34), and thelength of the superframe 140 may be advantageously selected so that thenode A sends out measurement data to the node B during the timeslot 142(TS0) once during each of the cycles 150-154. In case the wirelessnetwork 14 defines 10 millisecond timeslot, the node A sends data to thenode B once every 30 milliseconds. If, however, the node A isreconfigured to report measurements once every 50 milliseconds, thenetwork manager 27, alone or in cooperation with the node A, mayreconfigure the frame 140 to have a length of five timeslots instead. Inother words, the length of each superframe may reflect a particulartransmission requirement of a particular network device 25A-B or 30-50.

On the other hand, more than one network device 25A-B or 30-50 may use asuperframe for transmitting or receiving data. Referring again to FIG.4, the node B (the network device 60) may regularly transmit data to thenode C (the network device 36) in the timeslot 144 of the superframe140, although the superframe 140 may be primarily associated with thenode A. Thus, different timeslots of a particular superframe may be usedby different network devices to originate, route, or receive data. In asense, the timeslots of each superframe may be understood as a resourceallocated to different devices, with a particular priority assigned tothe device that “owns” the superframe. Further, it will be appreciatedthat each network device may participate in multiple superframes. Forexample, the network device 34 in FIG. 4 may route data on behalf ofother network devices (e.g., the network device 32 illustrated in FIG.1), in addition to propagating its own data via the router device 60.Preferably, a device participating in multiple superframes does notschedule simultaneous communications in different superframes. Whileonly three superframes are illustrated in FIG. 6, the wireless network14 of FIG. 1 may include any number of superframes, with each of thedifferent superframes having any desired or useful length based on thetypes and frequencies of communication being performed in or betweenparticular devices and set of devices.

As indicated above, the ASN counter 68 (see FIG. 1) may reflect thetotal number of timeslots consecutively scheduled since the activationof the wireless network 14. In other words, only those timeslots whichoccur following another timeslot affect the ASN count, and the number ofconcurrently scheduled superframes has no impact on the ASN value. Tofurther outline the operation of the ASN counter 68, FIG. 7 illustratesa schedule 250 including several concurrent superframes 252-256 createdat or after a network start time 260. The superframe 252 may be afour-timeslot superframe in which the relative slot numbers iterate fromzero to three. Similarly, the superframe 254 may similarly start at thenetwork start time 260 but include eight timeslots numbered zero throughseven. On the other hand, the superframe 256 may be created at a latertime when a new network device joins the wireless network 14, forexample, or when the network manager 27 allocates temporary resourcesfor a special purpose such as to accommodate a block mode transfer. Thevalues which the network manager 27 may assign to the ASN counter 68during the operation of the network schedule 250 are generally indicatedas a sequence 270. It will be noted that the value of the ASN counter 68increases with every new timeslot irrespective of a superframe withwhich the timeslot is associated.

Referring back to FIG. 1, each of the network devices 25A-B and 30-50may maintain a local copy of the ASN counter 68. During operation of thewireless network 14, the gateway device 22 may propagate the currentvalue of the ASN counter 68 to each network device 25A-B or 30-50 fornetwork synchronization. Every network device 25A-B or 30-50 may thencompare a local copy of the ASN counter to the value reported in a datapacket sent by the gateway device 22 and, if necessary, update the localcopy to match the value of the ASN counter adjusted according to apropagation delay of the message. For example, the network schedule 67may specify that the network node 32 receives a certain type of a datapacket, originated by the gateway device 22 and associated with aparticular superframe, in a third timeslot following the timeslot inwhich the gateway device 22 transmits the packet to a neighbor device.The network node 32 may accordingly check whether the current ASN valuestored by the network node 32 is indeed the value of ASN included in thedata packet plus three (i.e., the number of timeslots scheduled sincethe gateway device 22 sent out the data packet).

It will be further noted that by propagating ASN information alongmultiple paths to each network device 25A-B and 30-50 (FIG. 1), thewireless network 14 ensures that as some of the direct wirelessconnections 65 encounter obstacles or fail for other reasons, thenetwork device 25A-B and 30-50 typically have at least one more accessto synchronization information, thus increasing the stability of thewireless network 14 and improving its overall resilience.

Additionally or alternatively, the network devices 25A-B and 30-50 alsouse the ASN value included in a data packet for ascertaining an age ofthe data packet. For example, a destination network node may receive adata packet, subtract the ASN inserted into the data packet at theoriginating network node from the local copy of the ASN value, andcalculate the age of the data packet by multiplying the difference inthe number of timeslots by the duration of an individual timeslot. Itwill be noted that by relying on the ASN value included in data packet,the wireless network 14 may enforce time-to-live (TTL) requirements,perform network diagnostics, collect delivery delay statistics, etc.

In some embodiments, every message between a pair of neighbor devicesmay include the ASN value in a Network Protocol Data Unit (NPDU). If thewireless network 14 uses the WirelessHART protocol 70 schematicallyillustrated in FIG. 2, each frame associated with the layer 78 mayinclude the ASN value to ensure that the neighbors sharing a directwireless connection 65 are properly synchronized. In one particularembodiment, each network device 25A-B or 30-50 may include only aportion of the ASN value in an NPDU frame to reduce the amount of datatransmitted at the level of the network layer protocol. Morespecifically, the wireless network 14 may maintain a 32-bit ASN valuebut the corresponding ASN snippet may include only the lower 16 bits ofthe ASN value. It will be appreciated that because a typical message isdelivered within a seconds or even milliseconds, several lower bits ofthe ASN value may be sufficient to measure the TTL value. Of course,other embodiments may use an even smaller snippet.

Further, the network devices 25A-B and 30-50 may use the ASN value todetermine a current timeslot in a particular superframe. In someembodiments, these devices may apply the following function to calculatea relative slot number within a superframe:relative slot number=ASN % (length of the superframe),where the symbol “%” represents the modulo division function. A networkdevice 25A-B or 30-50 may use this formula to construct an ordered listof the timeslots that are about to occur in the relevant superframes. Itwill be noted that in some embodiments, each new superframe of a certainlength may start at such a time as to fit an integer number ofsuperframes of this length between this time and the start time of thenetwork. Referring again to FIG. 7, for example, the superframe 256 mayhave eight timeslots and may accordingly start a timeslot 0, 8, 16, . .. , 8n, where n is an integer. In other embodiments, new superframes maynot start at an ASN value equal to a multiple of the superframe length,and the joining device may add an additional offset to a result ofapplying the formula above.

In another embodiment, the devices attempting to join the wirelessnetwork 14 may use the ASN value to properly synchronize with theactivate network schedule 67. In particular, each active network device25A-B and 30-50 may periodically sent out advertisement packets whichthe potential new neighbors of these devices may process to determinewhether one or more new direct wireless connections 65 may be formedbetween the joining device and one more of the advertising devices. Inaddition to evaluating the strength and, optionally, the quality of asignal associated with each advertising (potential) neighbor, thejoining device may consider a number of other factors when processingadvertisement packets. For example, each advertisement packet mayinclude a network identity field which the joining device may compare tothe network identity with which the joining device has been previouslyprovisioned. This process may ensure that the joining device joins thecorrect network if several similar wireless networks 14 operate within ashort distance from each other or if there is some overlap between thegeographical areas covered by these networks.

Referring to FIG. 8, a state diagram 300 illustrates some of therepresentative states in operation of a device joining the wirelessnetwork 14. In a state 302, the joining device may search for thepotential neighbors by listening to advertisement packets. In at leastsome of the embodiments, the joining device may not yet know the networkschedule 67 or even the relevant parts of the network schedule 67 butmay be aware of at least some of the channels on which the wirelessnetwork 14 operates (e.g., a set of carrier radio frequencies). In otherembodiments, the joining device may not be provisioned with any specificchannel information and may only store the network identity, asdiscussed above. In these cases, the joining device may initiallyconsider the entire unlicensed part of the radio spectrum as includingpotential communication channels of the wireless network 14. Whensearching for advertisement packets, the joining device may operate itstransceiver in the receive mode and scan each of the potential channelsfor advertisement packets according to any suitable algorithm. Forexample, the joining device may listen to each potential channel for aseveral seconds before transitioning to the adjacent channel.Alternatively, the joining device may transition to the adjacent channelafter each timeslot.

The state machine 300 may remain in the state 302 until the joiningdevice receives an advertisement packet. Next, the joining device mayprocess the received advertisement packet in a state 304. If, forexample, the strength and/or quality of the carrier signal associatedwith the reception of this advertisement packet is below a predefinedthreshold, the joining device may return to the state 302 and wait foradvertisement packets from other potential neighbors. Alternatively, thejoining device may not implement an absolute threshold and may onlyevaluate the relative strength of each potential direct wirelessconnection 65. On the other hand, the joining device may transition to asynchronization state 306 if the signal is acceptable.

In the state 306, the joining device may extract the ASN value from theadvertisement packet and update the local copy of the ASN count. In someembodiments, the joining device may transition back to the state 304 tocollect additional advertisement packets and to collect statisticsrelated to ASN values reported from various potential neighbors. In onesuch embodiment, the joining device may receive data packets of severaltypes, some of which may include ASN timestamps, i.e., fields storingASN values recorded at the time a source network device originated thecorresponding data packet. In addition to processing the ASN valuesreported in the received data packets, the joining device may use thedata packets to adjust the timing of a timeslot (e.g., start time, endtime, etc.) As discussed above, the joining device may then use the ASNvalue to determine relative slot numbers in one or more superframes ofthe network schedule 67. Thus, when the joining device receivesinformation regarding the available superframes in which the joiningdevice may transmit and receive data as part of a join and/orauthentication sequence, the joining device may quickly determine therelevant relative slot numbers by applying the modulo formula describedabove.

Upon completing synchronization, the joining device may begin tonegotiate admission to the wireless network 14 in a state 308. It willbe noted that the joining device may further transition through severaladditional states to complete authentication, negotiate bandwidthallocation, receive routing information from the network manager 27,obtain the scheduling of the management superframe, etc.

In another aspect, some of the network devices 30-50 may rely on the ASNvalue for quick re-synchronization after a period of dormancy. Ingeneral, while a network device may disconnect from the network andconnect again through the complete process of receiving advertisementinformation from a potential neighbor and accepting the invitation, itmay be desirable in some cases to avoid disconnecting a network devicefrom the wireless network but 14 rather to maintain the network devicein a dormant state. For example, the device 30 may collect measurementsvery rarely and may therefore have a very low update rate. Moreover,this network device may not be an intermediate hop for any of thenetwork devices (i.e., this device may not be designated as the nextnode on any of the graphs of any of the neighbors of the device) andthus may not be an active participant in data exchange. In this case,the network device 30 may be configured either by the network manager 27or manually by an administrator to request a long superframe for publishthe process data.

During the long intervals during which the network device 30 is notcommunicating, the network device 30 may go into a sleep mode toconserve battery life. Immediately prior to this next scheduledcommunication time, the network device 30 may wake up, listen to networktraffic to re-synchronize with the network and obtain the currentabsolute slot number. Thereafter, the network device 14 may wait for theproper relative slot in the current superframe, perform the scheduledcommunication at the appropriate time (i.e., during the scheduledrelative slot number(s), and return to the dormant state. Importantly,the network device 30 may thus avoid the time- and resource-intensiveprocess of leaving and joining the wireless network 14. Instead, thenetwork device 14 may efficiently reconnect by waking up according tothe period corresponding to the size of the requested superframe, andre-synchronize with the network in time for the next scheduledcommunication at the network device 14. In addition to saving power, thenetwork device 15 may thereby reduce the rate of transmission andminimize the amount of non-essential network traffic.

Additionally, the network device 25A-B and 30-50 may use ASN informationto resolve collisions when trying to access shared resources. Inparticular, multiple devices may sometimes share a certain link resource(i.e., a timeslot of a particular superframe occurring on particulardirect wireless connection 65) and may accordingly require a method ofresolving conflicts when two or more of these devices attempt totransmit over the shared link at the same time. In general, the wirelessnetwork 14 may manage shared links in a manner similar to the well-knownslotted aloha algorithm, and the network devices 25A-B and 30-50 may usea collision-avoidance scheme with an exponential backoff. However, thebackoff may be implemented as a delay measured in an integer number oftimeslots rather than in traditional units of time. Thus, a networkdevice 25A-B or 30-50 may back off by one timeslot, two timeslots, fourtimeslots, etc. By synchronizing backoff intervals with timeslots, thenetwork device 25A-B or 30-50 may optimize the retry mechanism andensure that retry attempts happen only when there is an actualpossibility of transmission. Meanwhile, shared links may be desirablewhen bandwidth requirements of some of the network devices 25A-B or30-50 are low, or when traffic is irregular. In many situations, usingshared links may decrease latency because a certain network device 25A-Bor 30-50 does not need to wait for a dedicated links and may insteadattempt to reserve the next available shared link.

In another aspect, the wireless network 14 may provide synchronizationcorrection to the network devices 30-50 so that each network device mayproperly identify the start of each timeslot. In particular, a networkdevice 30-50 may note the arrival of a data link protocol data unit(DLPDU) and, based on the information included in the DLPDU, calculatethe difference, Δt, between the time of arrival and the “ideal” time atwhich the network device 30-50 expected to see the arrival of the DLPDU.More precisely, the network device 30-50 may associated the time ofarrival with a reception of the complete start delimiter which framesthe DLPDU. The start delimiter may be, for example, a predefinedsequence of bits. In some embodiments, the network device 30-50 mayreport the calculated Δt value in every ACK or NACK sent in reply to aDLPDU received from a peer device. Of course, the value of Δt may bepositive or negative. In this manner, every communication transaction inthe wireless network 14 may measure the alignment of network timebetween the network devices 30-50, thereby significantly improving theability of these network devices 30-50 to properly keep network time. Insome embodiments, time synchronization may be based either on thearrival time of a DLPDU or on the Δt in the positive or negative datalink layer acknowledgements (ACK/NACK), depending on which deviceinitiates the transaction.

In an embodiment, the network manager 27 may designate, for each networkdevice 30-50, certain neighbors as time synchronization sources. If, forexample, the network devices 50, 30, and 34 are neighbors of the networkdevice 32, the network manager 27 may designate the network device 30 tobe one of the synchronization sources of the network device 32. Inoperation, when the network device 32 receives a DLPDU from the networkdevice 30, the network device 32 may adjust the local network time. Morespecifically, the device designated as the time synchronization sourcemay calculate the Δt value and include the calculated Δt value in a datapacket to the device dependent on the time synchronization source device(e.g., in an acknowledgement DLPDU). The time-dependent device may thenadjust the local time according to the Δt reported by the timesynchronization source. The Δt value may be measured in microseconds,although other scales (e.g., nanoseconds, seconds) are alsocontemplated. It will be noted, moreover, that the network device 30 maynot necessarily adjust the local network time in view of a DLPDU fromthe network device 30. In other words, the relationship between thenetwork devices 30 and 32 with respect to time adjustment need not besymmetrical.

In general, device designers should understand time driftcharacteristics of their products. Thus, should the time source of anetwork device begin to drift, the network device may transmit one ormore keep-alive DLPDUs, as needed, to the time synchronization neighborsto ensure that time synchronization is properly maintained. In anotheraspect, keep-alive DLPDUs also may be used for neighbor discovery and toconfirm the viability of quiescent links.

With respect to selecting time sources, several approaches arecontemplated. For example, the network manager 27 may define a path fortime propagation from the gateway 22 outward to the end network deviceof the wireless network 14. Conversely, the network manager 27 maydirect time synchronization in the opposite direction. In either case,the network manager 27 may direct time propagation along asynchronization chain so as to avoid forming closed loops. To take onespecific example, the network manager 27 may define one time sourcepropagation path as a originating at the gateway device 22, propagatethrough the network access point 25A, further propagate to the networkdevice 32, the network device 60, and finally to the network device 34.The network access point 25A may thus serve as a direct synchronizationsource of the network device 32 and as an indirect synchronizationsource of the network devices 60 and 34. In this example, the networkdevice 60 may have several neighbors (e.g., network device 36, etc) butmay use only the network device 32 for time synchronization. On theother hand, the network device 32 may not “trust” the timing of thenetwork device 60 and may instead defer to the network access point 25A.

From the foregoing, it will be appreciated that the network devices30-50 may maintain network time synchronization by sending DLPDUs andacknowledgements and by doing so, the network device 30-50 may alsotrack the absolute number across the entire wireless network 14. Allcommunications are scheduled to occur within a superframe duringspecific time slots in superframes. All devices must support multiplesuperframes and a specific superframe has one or more links. The linksspecify the slot and associated information required to pass on oraccept a packet.

Although the forgoing text sets forth a detailed description of numerousdifferent embodiments, it should be understood that the scope of thepatent is defined by the words of the claims set forth at the end ofthis patent. The detailed description is to be construed as exemplaryonly and does not describe every possible embodiment because describingevery possible embodiment would be impractical, if not impossible.Numerous alternative embodiments could be implemented, using eithercurrent technology or technology developed after the filing date of thispatent, which would still fall within the scope of the claims.

What is claimed:
 1. A method for synchronizing communications inwireless mesh network operating in a process control environment andincluding a plurality of network devices, comprising: defining acommunication timeslot of a predetermined duration, wherein each of theplurality of network devices transmits or receives data only within thecommunication timeslot; generating a network schedule including at leastone superframe having repeating superframe cycles each having a numberof communication timeslots sequentially numbered relative to a beginningof each cycle, the number of communication timeslots defining a lengthof the at least one superframe; maintaining an absolute slot numberindicative of a number of communication timeslots elapsed since a starttime of the wireless mesh network, including incrementing the absoluteslot number by one with each occurrence of a new timeslot; synchronizingeach of the plurality of network devices with respect to a timing of anindividual communication timeslot; and synchronizing each of theplurality of network devices with the network schedule based on theabsolute slot number.
 2. The method of claim 1, wherein synchronizingeach of the plurality of network devices with the network scheduleincludes transmitting the absolute slot number in each network layerprotocol data unit.
 3. The method of claim 1, wherein synchronizing eachof the plurality of network devices with respect to the timing of theindividual communication timeslot includes: synchronizing each of theplurality of network devices with respect to a communication timeslottiming to calculate a start time of a next scheduled communicationtimeslot, including: calculating, at each of the plurality of networkdevices, an expected time of arrival of a data packet; receiving thedata packet at an actual time of arrival; calculating the differencebetween the expected time of arrival and the actual time of arrival; andcalculating a corrected start time of a next scheduled communicationtimeslot based on the calculated difference.
 4. The method of claim 3,further comprising adjusting a synchronization of the each of theplurality of network devices, including sending the calculateddifference between the expected time of arrival and the actual time ofarrival in an acknowledgement packet to a neighbor network device,wherein the acknowledgement packet is associated with a data linkprotocol layer.
 5. The method of claim 1, wherein synchronizing each ofthe plurality of network devices with the network schedule includestransmitting a portion of the absolute slot number in each network layerprotocol data unit, wherein the portion of the absolute slot numberincludes several least significant bits of the absolute slot number. 6.The method of claim 1, wherein generating a network schedule furtherincludes creating a plurality of concurrent superframes, each superframehaving a different length; wherein each of the plurality of networkdevices communicates in at least one of plurality of concurrentsuperframes; and wherein synchronizing each of the plurality of networkdevices with the network schedule includes obtaining a relative slotnumber in at least one of the plurality of superframes associated withthe each of the plurality of network devices.
 7. The method of claim 6,wherein synchronizing each of the plurality of network devices with thenetwork schedule includes applying a formularelative slot number=ASN % superframe length; wherein relative slotnumber is the relative slot number in the at least one of the pluralityof superframes associated with the each of the plurality of networkdevices, ASN is the absolute slot number, % is a modulo divisionoperation, and superframe length is the length of the at least one ofthe plurality of superframes associated with the each of the pluralityof network devices.
 8. The method of claim 1, further comprisingsynchronizing a joining device attempting to join the wireless meshnetwork based on the ASN value, including: transmitting an advertisementmessage from at least one of the plurality of network devices; includingthe absolute slot number in the advertisement message; receiving theadvertisement message at the joining device; and calculating a relativeslot number within the at least one superframe of the network schedulebased on the absolute slot number at the joining device.
 9. The methodof claim 1, wherein maintaining the absolute slot number includesindependently maintaining a copy of the absolute slot number at each ofthe plurality of network devices.
 10. The method of claim 9, whereinmaintaining the absolute slot number further includes maintaining aglobal absolute slot number at a network manager responsible forgenerating the network schedule.
 11. The method of claim 1, wherein thewireless mesh network includes a plurality of multi-hop communicationpaths connecting pairs of the plurality of network devices; and whereinsynchronizing each of the plurality of network devices with the networkschedule includes transmitting the absolute slot number along at leasttwo distinct paths, wherein the two distinct paths differ in at leastone hop.
 12. A method for synchronizing communications in wireless meshnetwork operating in a process control environment and including aplurality of network devices, comprising: defining a communicationtimeslot of a predetermined duration for transmitting or receiving dataat at least one of the plurality of network devices; continuouslyscheduling non-overlapping adjacent communication timeslots duringoperation of the wireless mesh network; maintaining an absolute slotnumber counter indicative of a number of communication timeslots of apredetermined duration elapsed since a formation of the wireless meshnetwork; and synchronizing communications between the plurality ofnetwork devices based on the absolute slot number counter.
 13. Themethod of claim 12, wherein maintaining the absolute slot number counterincludes: setting the absolute slot number counter to zero at theformation of the wireless mesh network; and incrementing the absoluteslot number counter by one with each occurrence of a new communicationtimeslot.
 14. The method of claim 12, wherein maintaining the absoluteslot number counter includes maintaining a global absolute slot numberat a network manager responsible for scheduling communications in thewireless mesh network; and wherein synchronizing communications betweenthe plurality of network devices based on the absolute slot numbercounter includes: propagating the absolute slot number counter to eachof the plurality of network devices; and updating a copy of the absoluteslot number counter at each of the plurality of network devices to matchthe global absolute slot number.
 15. The method of claim 14, wherein thenetwork manager resides in a gateway device connecting the wireless meshnetwork to a plant automation network; and wherein propagating theabsolute slot number counter to each of the plurality of network devicesincludes propagating the absolute slot number counter from the gatewaydevice.
 16. The method of claim 12, wherein at least some of theplurality of network devices are field devices performing a processcontrol function; and wherein synchronizing communications between theplurality of network devices includes: transmitting an update from oneof the field devices according to a scheduled update rate of the fielddevices; transitioning the field device in a sleep mode, wherein thefield device does not receive or transmit data in the sleep mode;transitioning the field device from the sleep mode immediately prior toa next scheduled update; and synchronizing the field device with thewireless mesh network, including: matching a value of the absolute slotnumber counter stored at the field device with a value maintained by thewireless mesh network; and calculating a time during which the fielddevice is scheduled to transmit data based on the matched absolute slotnumber counter and the update rate of the field device.
 17. A wirelessmesh network operating in a process control environment, comprising: anabsolute slot number counter that stores a number of communicationtimeslots of a predefined duration elapsed since a formation of thewireless mesh network, the number of communication timeslots incrementedby one with each occurrence of a new timeslot; a plurality of wirelessfield devices, each communicating with at least another one of theplurality of field devices according to a device specific schedule andincluding: a memory unit storing a definition of an update superframeassociated with the field device, wherein the update superframe includesa repeating sequence of superframe cycles, each cycle having a number ofcommunication timeslots sequentially numbered relative to a beginning ofeach cycle; and a processing unit that calculates a relative timeslotwithin the update superframe based on the absolute slot number counter;and a network manager that maintains each device specific schedule ofeach of the plurality of wireless field devices.
 18. The wireless meshnetwork of claim 17, further comprising: a plurality of direct wirelessconnections between pairs of the plurality of wireless field devices; aplurality of graphs each including at least one of the plurality ofdirect wireless connections and connecting a pair of the plurality ofwireless field devices; wherein the wireless mesh network propagates thea value of the absolute slot number counter to each of the plurality ofwireless field devices along at least two of plurality of graphs. 19.The wireless mesh network of claim 17, further comprising a gatewaydevice that operatively couples the wireless mesh network to an outsidenetwork and wherein the gateway device maintains the absolute slotnumber counter.
 20. The wireless mesh network of claim 17, wherein thenetwork manager is a software entity running in the gateway device. 21.A method for synchronizing communications in wireless mesh networkoperating in a process control environment and including a plurality ofnetwork devices, comprising: defining a communication timeslot of apredetermined duration, wherein each of the plurality of network devicestransmits or receives data only within the communication timeslot;generating a network schedule including at least one superframe havingrepeating superframe cycles each having a number of communicationtimeslots sequentially numbered relative to a beginning of each cycle,the number of communication timeslots defining a length of the at leastone superframe; maintaining an absolute slot number indicative of anumber of communication timeslots elapsed since a start time of thewireless mesh network, including incrementing the absolute slot numberby one with each occurrence of a new timeslot; and synchronizing each ofthe plurality of network devices with the network schedule based on theabsolute slot number.
 22. A method for synchronizing communications inwireless mesh network operating in a process control environment andincluding a plurality of network devices, comprising: defining acommunication timeslot of a predetermined duration, wherein each of theplurality of network devices transmits or receives data only within oneof a plurality of scheduled communication timeslots; designating a firstone of the plurality of network devices as a timeslot synchronizationsource of a second one of the plurality of network devices; sending afirst data packet from the first one of the plurality of network devicesto the second one of the plurality of network devices, including:sending a time adjustment value in the first data packet, the timeadjustment value based on an absolute slot number indicative of a numberof communication timeslots elapsed since a start time of the wirelessmesh network; and adjusting, based on the time adjustment value, atimeslot synchronization at the second one of the plurality of networkdevices.
 23. The method of claim 22, wherein the first data packet is anacknowledgement data packet; and wherein sending a time adjustment valuein the first data packet includes: calculating an expected time ofarrival of a non-acknowledgement data packet at the first one of theplurality of network devices, wherein the acknowledgement data packet issent in response to the non-acknowledgement data packet; detecting anactual time of arrival of the non-acknowledgement data packet;calculating a difference between the expected time of arrival and theactual time of arrival.
 24. The method of claim 22, further comprising:designating a third one of the plurality of network devices as atimeslot synchronization source of the first one of the plurality ofnetwork devices to form a synchronization chain including the first oneof the plurality of network devices, the second one of the plurality ofnetwork devices, and the third one of the plurality of network devices.25. The method of claim 24, wherein forming a synchronization chainincludes: associating a subset of the plurality of network devices withthe synchronization chain; and preventing a first one in the subset ofthe plurality of network devices from serving as a timeslotsynchronization source of another one of the subset of the plurality ofnetwork devices if the other one of the subset of the plurality ofnetwork devices is one of a direct or an indirect synchronization sourceof the first one in the subset of the plurality of network devices. 26.The method of claim 24, wherein the wireless mesh network includes agateway device connecting the wireless mesh network to an externalnetwork; and wherein forming a synchronization chain includesassociating the gateway device with a synchronization source of each ofthe plurality of network devices.